The study of the interrelationships between normal and progressing cells within a stratifying epithelial tissue has been hindered by the lack of suitable model systems. An organ culture model has been developed in which test cells can be place in direct contact with basal epithelial cells. Explants of oral mucosa are destratified in vitro and experimental cells are seeded onto them. When returned to proper culture conditions, the normal basal cells will redifferentiate which allows for examining cell-cell interactions occurring within a stratifying epithelial system. A unique series of murine epithelial cell lines at known stages along progression to malignancy (primary keratinocytes, initiated epithelial cells, progressed epithelial cells, carcinoma cells) will be used to seed isogeneic mucosal explants. Interactions between seeded and normal host cells will be examined. The ability of the seeded cells to compete with normal cells, repopulate the epithelial surface and/or invade into the stroma will be determined. By seeding cells embedded in or growing on collagen gels, a well defined tumor-host border will be produced. The effect of seeded cells on the proliferative kinetics (labeling index, mitotic index) and differentiative status of normal basal cells will be determined in relationship to distance from the seeded cells. By using a series of gels (cell embedded, mitomycin C inactivated, acellular) and either destratified or denuded mucosal explants, the effects of seeded cells on normal epithelial potentials and effects of normal cells on seeded cell potentials will be determined. The results from explant seedings will be compared to the in vitro and in vivo characteristics of these cell lines (normalcy of differentiation, ploidy, growth in soft agarose, invasive potential, tumorigenicity, metastatic potential); these parameters reflect the cell's state of malignancy or normality. Heparanase and type 1V collagenase levels will be examined as the starting point in understanding the biochemical mechanisms responsible for seeded cell potentials. These studies will not only provide information on the types of interactions which occur between normal cells and cells which are at different stages along the progression to malignancy, but will also demonstrate the significance of these interactions on epithelial dynamics and steady state parameters. The culture techniques are analogous for human tissue which will enable clinical studies to be undertaken for the characterization and staging of preneoplastic epithelial lesions.